Most actuators currently in use, whether linear or rotary, are based on magnetic forces. The prior art linear actuators (typically solenoids) and rotary actuators (typically motors) require the use of magnetic materials such as iron, causing them to be heavy and unsuitable for applications that require high-power actuators with light weight.
Motors also have the drawback that they operate efficiently at speeds of thousands of RPMs. Many applications require slow speed and high torque, and addressing these applications with electric motors requires additional gears, pulleys or other means to convert the motor's high speed and low torque into the desired low speed and high torque. Addition of gears adds cost, weight, and noise. Gearing also adds significant loading that prevents the output drive from moving freely when the motor is not powered. Some actuator applications, especially those involving muscle augmentation devices, require a low power mode in which the output of the actuator can move freely.
Other actuator applications, such as those based on hydraulics or pneumatics, can provide low speed high torque activation. However, they have severe drawbacks in portability, efficiency and weight due to the extra components needed to compress the working fluid.
Actuators based on electrostatics have been in limited use for over a century. The first electrostatic motors were demonstrated by Ben Franklin and others, but have never before been configured in a way that is suitable for high power applications. They did not find widespread use partially because high-power electrostatic actuators require high voltage electronic circuitry, fine geometries, and designs that are not prone to voltage breakdown.
Electrostatic actuators such as comb drives have been used as micro-actuators in recent devices built from silicon wafers. These actuators are of much smaller scale than the type described in the current invention, and the designs do not tend to scale directly to high power actuators.
One recent design of a high powered actuator was developed by a team led by Prof. Higuchi at the University of Tokyo. That team describes a high-powered linear electrostatic actuator that can lift significant weight. However, the fabrication of this actuator has several drawbacks. It is designed with three interleaved phases driven by high-voltage sine waves 120 degrees out of phase (see prior art FIG. 1). In order to produce high forces, the phase lines must be spaced very close together (a few thousandths of an inch). A single defect of a few thousandths of an inch, whether caused by manufacturing errors, contamination, or gradual breakdown of the dielectric material between the phases, causes the phases to arc over and generate heat that tends to destroy the device.
Another drawback of the Higuchi design is that it uses spaced thin phase conductors to generate the attractive force. The force is proportional to the gap between electrodes on the slider and stator, hence the force is very strong when attracting phases are close to each other, but the force drops off rapidly when they are separated. This effect causes “torque ripple” in which the forces have strong peaks and valleys. The Higuchi group has proposed a solution to torque ripple by skewing the electrodes, but the peak force is then reduced. Yet another drawback of the Higuchi design is that it requires the high voltage phases to be driven as sine waves. The circuitry for driving the phases with high voltage sign waves is more complex and less effective relative to the circuitry for driving digital signals. Circuitry designed to apply high forces with phases driven at binary high/low voltage levels is simpler and more effective.
Higuchi and others have also developed electrostatic induction motors with a moving electrode that has no connections but relies on induced charge that interacts with the phases driven by the stator electrodes. This type of motor is known to have much lower torque than one in which both rotor and stator have electrodes connected to high differential voltages.
FIG. 2 shows another example of the prior art from FIG. 7 of U.S. Pat. No. 6,525,446. This electrode structure shows drawbacks similar to Higuchi's linear actuator in the earlier example, but in a rotary structure. It has three phases that alternate around the stator, and grounded electrodes that alternate around the rotor. The three phases of stator electrodes, D1, D2 and D3, alternate around the circumference of the stator. The phases must be driven at high voltages in order to provide high forces, and must be spaced with insulated gaps to avoid arcing between the phases. The need for gaps works against the desire to increase the number of phase electrodes in order to increase torque. As the size of the electrodes shrinks, the gaps become a larger fraction of the total circumference. Wider gaps cause the torque to drop off sharply when the rotor is positioned at a point where there is a large distance between the rotor electrode and the active phase. Hence, the prior art actuators suffer from uneven torque and reduced maximum torque. Another drawback of the actuator design in FIG. 2 is the difficulty of interconnecting all like phase lines together. One phase can be connected at the outer diameter and another phase connected at the inner diameter. However, interconnecting the third phase requires each phase line to have a feedthrough or crossover to avoid shorting the third conductors to one of the other phases. With fine geometries, it may be impossible or very costly to include a plated through hole or crossover for every electrode of the third phase. Hence such actuators require a more expensive manufacturing technique than ones in which the intra-phase connections can be made in the same plane. The undesirable crossovers are evident in the center of the stator in FIG. 2.
Another drawback of the prior art is the need to drive the phases with complex, high-voltage analog waveforms. Higuchi drives the phases with sine waves, and U.S. Pat. No. 6,525,446, drives them with a voltage dependent on the current rotor position. In both cases, the circuits driving the phases need the ability to set phases to arbitrary voltages levels. This requires a more complex circuit than one that generates digital high-voltage pulses by switching the phases between maximum and minimum levels.